The present invention relates in general to step generators utilizing diode switches to switch load currents through a load resistor to produce a stepped output voltage, and more particularly, to step generators having constant edge parameters while driving variable loads.
One method of utilizing diode switches to switch loads currents through a load is shown in FIG. 2 of U.S. Pat. No. 4,758,736 to Agoston, et al. The circuit of FIG. 2 shows two current switches each having two Schottky diodes. Depending upon the relative values of V1 and Vo, and V2 and Vo, the currents I1 and I2 are either steered towards or away from the load Ro. This arrangement allows abrupt transitions in the output voltage. However, a varying load Ro will produce varying delays and slight ringing in the output step. The output voltage Vo varies directly with the value of the output load Ro, for a given constant load current. This changing voltage changes the switching threshold value of the two diode switches. Thus, depending upon the load Ro, the output step is generated at various points along the input waveform, resulting in delay variations. Further, the capacitance of the diodes D2 and D4 coupled to the output load Ro results in a slight overshoot as the switching threshold is varied and more voltage is stored across the diodes in the reverse biased state.
What is desired is an output step generator which can produce an output step having nearly constant edge parameters regardless of the value of the load being driven.